1. Field of the Invention
This invention relates generally to the treatment of spent sulfuric acid streams formed in the manufacture of alcohols via the H.sub.2 SO.sub.4 catalyzed hydration of olefins.
2. Description of the Prior Art
Large volumes of alcohols are produced annually by the catalytic hydration of olefins, in which the selected olefin feed is absorbed in a concentrated sulfuric acid stream to form the corresponding alkyl ester of the sulfuric acid. Thereafter, water is admixed with the ester-containing liquid to hydrolyze the ester and to form the desired alcohol which is then recovered, generally by stripping with steam or other heating fluid. There is thereby produced a diluted sulfuric acid stream which must, for economic reasons, be treated to concentrate it with respect to its H.sub.2 SO.sub.4 content after which it is recycled to the absorption step.
Organic impurities in these various sulfuric acid streams accumulate due to this continuous acid recycle, and this accumulation results in the deposit of carbonaceous materials on the inner surfaces of process equipment. These carbonaceous deposits, which result from the thermal degradation ("coking") of the organic impurities, can foul equipment, and severely reduce the flow rate of liquids therethrough. Removal of these deposits is, therefore, periodically necessary and requires shutting down of the facilities and physical removal of these deposits, as by manually scraping the fouled surfaces. This involves considerable expense in manpower and plant down-time, and results in a significant loss of overall annual plant capacity. In addition, the carbonaceous deposits which are thus removed are waste materials which create still further expense, and attendant environmental problems, in the need to safely dispose of these materials.
Various methods have been developed for electrolytic regeneration of spent alkylation sulfuric acids, such as are disclosed in Canadian Pat. No. 586,878 and U.S. Pat. Nos. 2,744,861, 2,793,180, 2,793,181 and 2,793,182.
U.S. Pat. No. 3,917,521 claims an improvement in prior art processes for regeneration of spent alkylation acids employing a method of sulfurless electrolysis in which persulfate ions are either supplied externally or generated in situ to permit the use of Pt electrodes in the electrolysis.
U.S. Pat. No. 3,616,337 relates to an electrochemical process for purification and concentration of spent sulfuric acids. In electrolysis of spent alkylation acids, water is added in controlled amounts to dilute the water content to between 1 and 2 times that amount of water stoichiometrically required to react with the carbon present in the spent acid, based on the reaction equations disclosed by the patentee. The patent also suggests, but it gives no working embodiment of, the application of his electrochemical method to dilute spent sulfuric acids containing about 40 to 50% water and about 1 to 2% by weight of carbon (10,000 to 20,000 ppm carbon), obtained from the manufacture of isopropyl alcohol. However, the patentee indicates that these acids should be distilled prior to electrolysis to remove water which is present in excess of the patentee's required range of from 1 to 2 times stoichiometry. Since a 1 to 2 wt% carbon content would require from about 3 to 12 wt% water, according to the patentee' s method the latter acids are required to be concentrated from the original 48 to 59 wt% sulfuric acid to a range of from 86 to 96 wt% sulfuric acid prior to electrolysis.
U.S. Pat. No. 4,085,016 is directed to electrolytic treatment to decolorize sulfuric acid streams which are formed from sulfur dioxide obtained in the roasting of sulfide ores. A portion of a concentrated sulfuric acid stream which contains from 1 to 200 ppm carbon as organic impurities is diverted, diluted with water, cooled to a temperature of from 5.degree. to 20.degree. C. and then electrolyzed to form a predetermined quantity of peroxosulfuric acids. The resulting stream is then returned to the main acid stream to oxidize the organic material therein. In accord with the patentees' required electrolysis temperatures, and cited by the patentees, is C. L. Mantell, Electrochemical Engineering, pages 326-328 (4th edition, 1960), in which both current efficiency and the yield of persulfuric acid (H.sub.2 S.sub.2 O.sub.8) is shown to significantly decrease at electrolysis temperatures above 20.degree. C., and the yield is said to be practically zero at 60.degree. C., with the persulfuric acid being said to decompose as fast as it is formed.
Spent sulfuric acid streams produced in different processes differ widely in their impurity-content and in their critical process characteristics. Thus, the above methods are not readily adaptable for use in treating spent acid streams formed in dissimilar processes, such as in the hydration of olefins to prepare alcohols.
Belgian Pat. No. 437,920, as abstracted in 36 Chem. Abs. 2969.sup.8 (1942) absorbed ethylene with H.sub.2 SO.sub.4, diluted the extract liquid and then distilled to remove ethanol and subjected the resulting dilute acid to electrolysis in such a manner as to yield a more concentrated acid at the anode and a more dilute acid at the cathode. The concentrated acid was reused for ethylene absorption and the dilute acid was used to dilute the extract liquid.